1. Technical Field
The present invention relates to a DC motor, more particularly, to an apparatus and a method for controlling the number of rotations per minute of a brushless DC motor.
2. Description of the Related Art
A motor refers to a device that converts electrical energy into kinetic energy, which allows rotary or linear mechanical motion. With the expansion of its fields of application due to the widespread use of electronic equipment in the electric, electronic, and mechanical industries, the importance of the motor has increased as a critical driving source. In addition, with a rapid growth of the industries have been introduced the motors as driving sources with faster speeds and larger capacities
In particular, precision control motors provide accurate movements with a fast response time and a wide range of speed controls. These motors are operated based on control signals by repeating stop, start, and reverse actions, etc. The precision control motors are continually increasing in importance with the advance of technology and expansion of application fields such as advances in power electronics, advances in microcomputers, precision processing technologies, high performance permanent magnets, and advances in surface mounting technologies.
Examples of such precision control motors include stepping motors, brushless DC motors, etc., which usually use permanent magnets with high performance.
The stepping motor is a type of electric motor that is used when something has to be positioned very precisely or rotated by an exact angle. Since the stepping motor provides digitalized control by using pulses, it is suitable for micoms. The stepping motor rotates by a specified angle and stops with a high level of precision without feedback for detecting the position of the shaft of the motor. Also, the stepping motor allows open loop control, is easily controlled through digital signals, and has a maintenance torque when stopping.
However, the torque is low in the stepping motors, so that it is difficult to apply in fields requiring high torque. Also, the stepping motor is likely to vibrate and resonate at certain frequencies, is weak against loads with inertia, and is likely to be stepped out during high-speed operations. Further, since a sufficient current cannot flow through winding wires due to an inductance effect of the winding wires during operation with a common driver, the torque is reduced with increasing pulse rate to yield a lower efficiency as compared to a DC motor.
Thus, a brushless DC motor, which provides high torque, is preferred in controlling the number of rotations per minute. Not only is the brushless DC motor durable and efficient, it allows for easy use in a constant speed control and a variable speed control.
The brushless DC motor is devised without a brush, which acts as a commutator, while maintaining the properties of a DC motor, and can be classified as a sensor type or a sensorless type according to whether or not it has a sensor that detects both the position of the rotor and the rotary speed. Such a brushless DC motor is employed in mobile devices such as mobile phones to allow the folder to implement automatic opening and closing function, or the camera module to implement automatic rotation.
Meanwhile, the rotational speed of the brushless DC motor is proportional to the voltage applied thereto. For example, when a 4V voltage is applied, the rotational speed is 6000 rpm, and when a 2V voltage is applied, the rotational speed is 3000 rpm. However, in order to perform the automatic functions, the brushless DC motor should maintain a constant rotational speed regardless of voltage decrease in the battery of the mobile device. Therefore, an integrated circuit for the driver with a low drop out regulator (LDO) has been employed to supply a constant voltage to the motor.
However, since the LDO takes up a space, it is unsuitable for small sized mobile devices. Therefore, there have been needs for a method maintaining the rotational speed of the motor without the LDO.